Thinners for invert emulsions

ABSTRACT

A method of reducing the viscosity of invert emulsions and oil-based drilling fluids and well service fluids comprising such emulsions over a broad temperature range is disclosed. The method comprises adding to said invert emulsions of the invention a non-ionic surfactant alone or in combination with a co-thinner having the formula: R″″—(C 2 H 4 O) n (C 3 H 6 O) m (C 4 H 8 O) k —H where R″″ is a saturated or unsaturated, linear or branched, alkyl radical having about 8 to about 24 carbon atoms, n is a number ranging from about 1 to about 10, m is a number ranging from about 0 to about 10, and k is a number ranging from about 0 to about 10. The non-ionic surfactant is a reaction product of ethylene oxide, propylene oxide and/or butylene oxide with C 10-22  carboxylic acids or C 10-22  carboxylic acid derivatives containing at least in position 9/10 and/or 13/14 structural units of general formula (I) where R 1  is a hydrogen atom or an OH group or a group OR 2 . R 2  is an alkyl group of about 1 to about 18 carbon atoms, an alkenyl group of about 2 to about 18 carbons atoms, or a group of the formula (II): R 3  is a hydrogen atom, an alkyl group of about 1 to about 21 carbon atoms or an alkylene group or 2 to 21 carbon atoms.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention is generally related to methods and compositions for drilling and servicing wellbores in hydrocarbon bearing subterranean formations. Particularly, this invention is related to oil-based drilling fluid systems comprising water-in-oil invert emulsions, and to thinners that enhance or enable use of such fluids over a broad temperature range.

2. Description of Relevant Art

A drilling fluid, or “mud” which a drilling fluid is also often called, is a specially designed fluid that is circulated in a wellbore as the wellbore is being drilled to facilitate the drilling operation. The various functions of a drilling fluid include removing drill cuttings from the wellbore, cooling and lubricating the drill bit, aiding in support of the drill pipe and drill bit, and providing a hydrostatic head to maintain the integrity of the wellbore walls and prevent well blowouts. Specific drilling fluid systems are selected to optimize a drilling operation in accordance with the characteristics of a particular geological formation.

A drilling fluid typically comprises water and/or oil or synthetic oil or other synthetic material or synthetic fluid (“synthetic”) as a base fluid, with solids in suspension. A non-aqueous based drilling fluid typically contains oil or synthetic as a continuous phase and may also contain water dispersed in the continuous phase by emulsification so that there is no distinct layer of water in the fluid. Such dispersed water in oil is generally referred to as an invert emulsion or water-in-oil emulsion.

A number of additives may be included in such oil based drilling fluids and invert emulsions to enhance certain properties of the fluid. Such additives may include, for example, emulsifiers, weighting agents, fluid-loss additives or fluid-loss control agents, viscosifiers or viscosity control agents, and alkali. Further general discussion and description of oil-based drilling fluids is provided in P. A. Boyd, et al., New Base Oil Used In Low Toxicity Oil Muds, Journal of Petroleum Technology, pages 137-142 (1985), which is incorporated herein by reference.

An essential criterion for assessing the utility of a fluid as a drilling fluid or as a well service fluid is the fluid's Theological parameters, particularly under drilling and wellbore conditions. For use as a drilling fluid, or as a fluid for servicing a well, the fluid must be capable of maintaining certain viscosities suitable for drilling and circulation in the wellbore. Preferably, a drilling fluid will be sufficiently viscous to be capable of supporting and carrying to the surface of the well drill cuttings without being so viscous as to interfere with the drilling operation. Moreover, a drilling fluid must be sufficiently viscous to be able to suspend barite and other weighting agents. However, increased viscosity can result in problematic sticking of the drill string, and increased circulating pressures can contribute to lost circulation problems.

Thinners may be added to the drilling fluid or drilling mud systems before and in the course of drilling. Anionic surfactants particularly from the group of the fatty alcohol sulfates, the fatty alcohol ether sulfates and the alkylbenzenesulfonates are examples of such thinners known in the prior art. Although such compounds have been shown to effect thinning of drilling fluids, their effectiveness as thinners is not always uniform over the entire range of temperatures (typically as low as about 40° F. (or lower) to as high as about 250° F. (or higher)) at which drilling fluids are used.

Thinners and other additives to drilling fluids, as well as drilling fluids employed in onshore and offshore wells, must commonly meet stringent environmental regulations related to biodegradability and toxicity. Further, drilling fluids and additives to drilling fluids must be able to withstand subterranean conditions that the fluids will typically encounter in a wellbore, such as high temperatures, high pressures, and pH changes.

A need exists for improved rheology-modifying or viscosity reducing additives to oil-based drilling fluids, and particularly to drilling fluids comprising invert (water-in-oil) emulsions, which are capable of being used over a broad range of temperatures. As used herein, unless indicated otherwise, a “broad temperature range” shall be understood to generally mean temperatures ranging from about 14° F. to about 350° F. and preferably ranging from about 40° F to about 250° F.

SUMMARY OF THE INVENTION

According to the method of the present invention, at least one compound or composition is added to a water-in-oil or invert emulsion, or a drilling fluid or well service fluid comprising such emulsion, which reduces the viscosity of the emulsion over a broad temperature range or which enables or enhances the ability of the emulsion to maintain its viscosity over a broad temperature range. The compound or composition, which may be generally called a “thinner,” continues to have this effect in a drilling fluid or well service fluid comprising the emulsion for use in drilling or servicing wellbores in subterranean formations, particularly hydrocarbon bearing subterranean formations, over a broad temperature range. The first such thinner compound of the present invention is a non-ionic surfactant which is a reaction product of ethylene oxide, propylene oxide and/or butylene oxide with C₁₀ carboxylic acids or C₁₀₋₂₂ carboxylic acid derivatives containing at least one double bond in position 9/10 and/or 13/14 having units of the general formula:

where R₁ is a hydrogen atom or an OH group or a group OR₂, where R₂ is an alkyl group of about 1 to about 18 carbon atoms, or an alkenyl group of about 2 to about 18 carbon atoms or a group of the formula:

where R₃ is a hydrogen atom, or an alkyl group of about 1 to about 21 carbon atoms or an alkylene group of about 2 to about 21 carbon atoms.

This first thinner compound may be used alone or may be used in combination with a second or other thinner or “co-thinner” compound having the following formula: R—(C₂H₄O)_(n)(C₃H₆O)_(m)(C₄H₈O)_(k)—H where R is a saturated or unsaturated, linear or branched, alkyl radical having about 8 to about 24 carbon atoms, n is a number ranging from about 1 to about 10, m is a number ranging from about 0 to about 10, and k is a number ranging from about 0 to about 10.

The invention also comprises an invert emulsion drilling fluid or well service fluid containing this first thinner compound, or containing said first thinner compound in combination with said second thinner compound.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph comparing yield point of mud systems with and without thinners of the invention tested as reported in Table 2.

FIG. 2 is a graph comparing yield point of mud systems with and without thinners of the invention tested as reported in Table 3.

FIG. 3 is a graph comparing yield point of mud systems with and without thinners of the invention tested as reported in Table 4.

FIG. 4 is a graph comparing yield point of mud systems with and without thinners of the invention tested as reported in Table 5.

FIG. 5 is a graph comparing yield point of mud systems with and without thinners of the invention tested as reported in Table 6.

FIG. 6 is a graph comparing yield point of mud systems with and without thinners of the invention tested as reported in Table 7.

FIG. 7 is a graph comparing yield point of mud systems with and without thinners of the invention tested as reported in Table 8.

FIG. 8 is a graph comparing yield point of mud systems with and without thinners of the invention tested as reported in Table 9.

FIG. 9 is a graph comparing yield point of mud systems with and without thinners of the invention tested as reported in Table 10.

FIG. 10 is a graph comparing yield point of mud systems with and without thinners of the invention tested as reported in Table 11.

FIG. 11 is a graph comparing yield point of mud systems with and without thinners of the invention tested as reported in Table 12.

FIG. 12 is a graph comparing yield point of mud systems with and without thinners of the invention tested as reported in Table 13.

FIG. 13 is a graph comparing yield point of mud systems with and without thinners of the invention tested as reported in Table 14.

FIG. 14 is a graph comparing yield point of mud systems with and without thinners of the invention tested as reported in Table 15.

FIG. 15 is a graph comparing yield point of mud systems with and without thinners of the invention tested as reported in Table 16.

FIG. 16 is a graph comparing yield point of mud systems with and without thinners of the invention tested as reported in Table 17.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention provides a method of influencing the rheology, and particularly reducing the viscosity, of invert (water-in-oil) emulsions and drilling fluids or well service fluids comprising such invert (water-in-oil) emulsions. The method is particularly applicable to fluids for use in wellbores penetrating hydrocarbon bearing subterranean formations. Such drilling fluids and well service fluids typically comprise a continuous oil phase, water dispersed in the oil phase, solids insoluble in the drilling fluid or well service fluid suspended in the fluid, and various additives. As the term is used herein, an “invert emulsion” or an “oil-in-water emulsion” is understood to mean the liquid portion of a drilling fluid comprising an emulsion (excluding any solids). The term “invert emulsion drilling fluid” means the total sum of what is circulated as a drilling fluid.

In the method of this invention, certain non-ionic surfactants are added to the invert emulsion or oil based drilling fluid (or well service fluid) to “thin” or reduce the viscosity of the fluid or to enhance the ability of the fluid to maintain its viscosity or to resist increasing viscosity over a broad range of temperatures. The particular non-ionic surfactants are reaction products of ethylene oxide, propylene oxide and/or butylene oxide with C₁₀₋₂₂ carboxylic acids or C₁₀₋₂₂ carboxylic acid derivatives containing at least one double bond in position 9, 10, 13, or 14 (and preferably one or two double bonds in position 9/10 and/or 13/14) having structural units of the general formula (I)

where R₁ is a hydrogen atom, or an OH group, or a group OR₂. R₂ is an alkyl group of about 1 to about 18 carbon atoms, or an alkenyl group of about 2 to about 18 carbon atoms, or a group of the formula (II)

R₃ is a hydrogen atom, or an alkyl group of about 1 to about 21 carbon atoms, or an alkylene group of about 2 to about 21 carbon atoms.

The alkoxylation products may be prepared in accordance with the teaching of DE 39 23 394, which is incorporated herein by reference, by reacting the OH-containing carboxylic acid derivatives, for example, with ethylene oxide, propylene oxide and/or butylene oxide in the presence of an appropriate catalyst at temperatures between about 110 and about 200° C. and pressures between about 10⁵ Pa and about 2×10⁶ Pa.

Suitable starting materials for OH-containing C₁₀₋₂₂ carboxylic acids or C₁₀₋₂₂ carboxylic acid derivatives are unsaturated, naturally occurring and/or synthesizable C₁₀₋₂₂ carboxylic acids or derivatives thereof containing carboxylic acid radicals having at least one or two double bonds in position 9, 10, 13 and/or 14. Examples of such unsaturated carboxylic acid derivatives are 9-dodecenoic acid, 9-tetradecenoic acid, 9-hexadecenoic acid, 9-octadecenoic acid, 9-octadecenoic acid, 9,12-octadecadienoic acid, 9,12,15-octadecatrienoic acid, 9-icosenoic acid, 13-docosenoic acid, and mixtures containing a large amount (i.e., preferably at least about 60%) of such unsaturated carboxylic acids. As starting materials it is preferable to use carboxylic acids having about 16 to about 22 carbon atoms and at least one or two double bonds in position 9 and/or 13 or carboxylic acid mixtures containing at least a large amount (i.e., preferably at least about 80%) of carboxylic acids having about 16 to about 22 carbon atoms and at least one or two double bonds in position 9 and/or 13.

Further examples of suitable unsaturated carboxylic acid derivatives for use in preparing thinner compounds for use in the invention are unsaturated C₁₀₋₂₂ carboxylic esters, such as for example, unsaturated C₁₀₋₂₂ carboxylic acid alkyl esters with monohydric alcohols having about 1 to about 18 carbon atoms. Particularly appropriate are C₁₀₋₂₂ carboxylic mono-, di- and/or triglycerides containing unsaturated C₁₀₋₂₂ carboxylic acid radicals having at least one or two double bonds in position 9 and/or 13. Also suitable are esters of C₁₀₋₂₂ carboxylic acids with other polyols, such as for example ethylene glycol or trimethylolpropane.

Unsaturated C₁₀₋₂₂ carboxylic acid C₁₋₁₈ alkyl esters are obtainable by esterifying the corresponding unsaturated carboxylic acid or by transesterifying the corresponding mono-, di- and/or triglycerides with C₁₋₁₈ alkyl alcohols, such as, for example, methanol, ethanol, propanol, butanol, isobutanol, 2-ethylhexanol, decanol and/or stearyl alcohol. Examples of such unsaturated C₁₀₋₂₂ carboxylic acid C₁₋₁₈ alkyl esters are methyl palmitate, methyl oleate, ethyl oleate, isobutyl oleate, 2-ethylhexyl oleate and/or dodecyl oleate and/or C₁₀ carboxylic acid C₁₋₁₈ alkyl ester mixtures containing at least a large fraction (i.e., at least about 60%) of those C₁₀₋₂₂ carboyxlic acid C₁₋₁₈ alkyl esters whose carboxylic acid radicals have at least one or two double bonds in position 9 and/or 13, such as, for example, palm oil methyl ester, soya oil methyl ester, colza oil methyl ester and/or tallow fatty acid ethyl ester. Other suitable starting materials for preparing the alkoxylation products for use in the invention are fats and oils of natural origin whose carboxylic acid content is comprised predominantly of unsaturated C₁₀₋₂₂ carboxylic acids having at least one or two double bonds in position 9 and/or 13, such as, for example, olive oil, linseed oil, sunflower oil, soya oil, groundnut oil, cottonseed oil, colza oil, palm oil, lard and tallow.

Unsaturated C₁₀₋₂₂ carboxylic acids and/or C₁₀₋₂₂ carboxylic acid derivatives may be epoxidized to the OH-containing compounds, for example, by reaction with peracetic acid in the presence of acidic catalysts or with performic acid formed in situ from formic acid and hydrogen peroxide. The oxirane rings of the epoxidized carboxylic acids and/or carboxylic acid derivatives are subsequently cleaved open to form hydroxyl groups by reaction with hydrogen or protic compounds, such as water, straight-chain and/or branched-chain alkyl and/or alkenyl alcohols having about 1 to about 18 carbon atoms or straight-chain and/or branched-chain, saturated and/or unsaturated C₁₋₁₈ carboxylic acids. Other natural or synthetic compounds comprising epoxide-containing carboxylic acids or carboxylic acid derivatives, such as castor oil or hydrogenated castor oil, may also be used. The cleavage conditions are chosen such that the acid-derivative groups and acid groups present remain intact.

The reaction of epoxidized carboxylic acid derivatives and/or epoxidized carboxylic acids with protic compounds may be carried out, for example, in accordance with the processes described in DE 39 23 394.

The carboxylic acids and/or carboxylic acid derivatives obtained by cleaving the oxirane rings, containing carboxylic acid radicals having at least one OH group in position 9, 10, 13 and/or 14, are subsequently reacted by known industrial processes with ethylene oxide, propylene oxide and/or butylene oxide (preferably with ethylene oxide and/or propylene oxide).

Alkoxylation products obtained by alkoxylating compounds of the formula (I) in which R₁ is a group OR₂ and R₂ is a radical of the formula (II) are preferred. Compounds where the radical R₃ is an alkyl group of about 8 to about 16 carbon atoms, most preferably about 8 to about 10 carbon atoms, are also preferred.

Some non-ionic surfactant compounds that could possibly be used as thinners in accordance with the present invention are also described in WO98/19043 of Henkel Kommandiegesellschaft auf Aktien. That international application teaches use of the compounds in the oil and gas industry as cleaning agents, in contrast to the use of the present invention.

Used as thinners according to the method of the invention, the non-ionic surfactants of the present invention reduce the viscosity or lower the yield point of the drilling fluid to which they are added over a broad range of temperatures.

Example drilling fluids comprising invert (water-in-oil) emulsions of particular use in the method of the invention generally have an oil phase comprising diesel oil, paraffin oil and/or mineral oil, or a synthetic oil. Alternatively, other carrier fluids may be used such as carboxylic esters, alcohols, ethers, internal olefins, alphaolefins (IO and/or AO), and polyalphaolefins (PAO), which may be branched or unbranched but are preferably linear and preferably ecologically acceptable (non-polluting oils). Preferably, the oils or carrier fluids used for the oil phase of the drilling fluid will be comprised of compounds which are flowable and pumpable at temperatures above about 32° F. (about 0° C.) or as low as about 40° F. (about 5° C.) as well as at higher temperatures. For example, compounds selected from one or more of the following groups or classes below are believed particularly suitable to comprise the oil phase of drilling fluids used in the present invention:

-   -   (a) most preferably, carboxylic esters of the formula:         R′—COO—R″  (III)         -   where R′ is a saturated or unsaturated, linear or branched,             alkyl radical having about 1 to about 23 carbon atoms and R″             is an alkyl radical, branched or unbranched, saturated or             unsaturated, having about 1 to about 23 carbon atoms;     -   (b) also preferably, linear or branched olefins having about 8         to about 30 carbon atoms;     -   (c) water-insoluble symmetric or asymmetric ethers of monohydric         alcohols of natural or synthetic origin, said alcohols         containing about 1 to about 24 carbon atoms;     -   (d) water-insoluble alcohols of the formula:         R′″—OH  (IV)         -   where R′″ is a saturated, unsaturated, linear or branched,             alkyl radical having about 8 to about 24 carbon atoms; and     -   (e) carbonic diesters.

Such suitable oils are taught further, for example, in: European Patent Applications 0 374 671, 0 374,672, 0 382 070, and 0 386 638 of Cognis; European Laid-Open Specification 0 765 368 of Cognis (linear olefins); European Application 0 472 557 (water insoluble symmetric or asymmetric ethers of monohydric alcohols of natural or synthetic origin containing about 1 to about 24 carbon atoms); European Application 0 532 570 (carbonic diesters). Carboxylic esters of formula (III) above are preferred for the oil phase of drilling fluids used in this invention and particularly preferred are the esters described in European Laid-Open Specification EP 0 374 672 and EP 0 386 636.

In a preferred embodiment of this invention, non-ionic surfactants of the invention are added to drilling fluids comprising invert emulsions having an oil phase comprising esters of formula (III) where the radical R′ in formula (III) is an alkyl radical having about 5 to about 21 carbon atoms (or more preferably about 5 to about 17 carbon atoms or most preferably about 11 to about 17 carbon atoms). Particularly suitable alcohols for making such esters are branched or unbranched alcohols with about 1 to about 8 carbon atoms, for example, methanol, isopropanol, isobutanol, and 2-ethylhexanol. Alcohols having about 12 to about 18 carbon atoms may alternatively be preferred for making other esters suitable for the invention.

For example, additional preferred esters for the oil phase of drilling fluids used in the invention include, without limitation: saturated C12-C14 fatty acid esters and unsaturated C16-C18 fatty acids (with isopropyl-, isobutyl- or 2-ethylhexanol as the alcohol component); 2-ethylhexyl octanoate; acetic acid esters, especially acetates of C8-C18 fatty alcohols; branched carboxylic esters disclosed in WO 99/33932 of Chevron or EP 0 642 561 of Exxon; alpha olefin mixtures disclosed in EP 0 765 368 A1 of Cognis and Halliburton; and blends of these various esters.

The oil phase of the emulsions of the drilling fluids used in the invention is preferably comprised of at least about 50% by volume of one or more preferred compounds (a)-(e) above. More preferably, such preferred compounds comprise about 60% to about 80% by volume of said oil phase, and most preferably, such preferred compounds comprise about 100% of the oil phase.

Water is preferably present in the liquid phase of the emulsions of the drilling fluids used in the invention in amounts preferably not less than about 0.5% by volume (excluding solids in the liquid phase). In a preferred embodiment of this invention, the nonionic surfactant thinners of the present invention are added to drilling fluids (preferably comprising invert emulsions) containing about 15% to about 35% by volume water and more preferably about 20% by volume water and about 80% by volume oil phase.

To compensate for the osmotic gradient between the drilling mud and the formation or connate water, water in drilling fluids used in the present invention typically includes fractions of electrolytes, such as calcium salts and/or sodium salts. CaCl₂ in particular is frequently used, although other salts from the group of alkali metals and/or alkaline earth metals are also suitable, with potassium acetates and formates being common examples.

Preferred drilling fluids used in this invention have the following rheology: plastic viscosity (PV) preferably in the range of about 10 to about 60 cP, and more preferably in the range of about 15 to about 40 cP, and yield point (YP) preferably in the range of about 5 to about 40 lb/100 ft², and more preferably in the range of about 10 to about 25 lb/100 ft², when measured at about 122° F. (about 50° C.). At lower temperatures, i.e., at or below about 40° F. (about 4° C.), the YP should not exceed about 75 lb/100 ft², and should preferably be in the range of about 10 to about 65 lb/100 ft², more preferably about 15 to about 45 lb/100 ft², and most preferably less than about 35 lb/100 ft². A preferred practicable lower limit for YP for drilling fluids used in this invention is about 5 lb/100 ft².

Methods for determining these parameters of PV and YP are well known to those skilled in the art. An example reference is “Manual of Drilling Fluids Technology”, particularly the chapter on Mud Testing, available from Baroid Drilling Fluids, Inc., in Houston, Tex. (USA) and Aberdeen, Scotland, incorporated herein by reference.

The solids content (not including low gravity solids), or amount of weighting agents, in drilling fluids used in this invention is preferably about 0 to about 500 lb/bbl, and most preferably about 150 to about 350 lb/bbl. The mud weight, i.e., the density of the drilling fluids, is preferably in the range of about 8 to about 18 lb/gal. and more preferably about 9 to about 15 lb/gal. Such solids, or weighting agents, which serve to increase density of the drilling fluids, may be any solids known to those skilled in the art as useful for such purpose, but will preferably be inert or environmentally friendly. Barite and barium sulfate are examples of commonly used weighting agents.

Drilling fluids used in this invention may optionally also contain other additives known to those skilled in the art, such as fluid-loss control additives and emulsifiers. Alkali may also be used, preferably lime (calcium hydroxide or calcium oxide), to bind or react with acidic gases (such as CO₂ and H₂S) encountered during drilling in the formation. Such alkali, or an alkali reserve, is known to prevent hydrolysis by acidic gases of generally acid-labile esters of the drilling fluid. Preferred quantities of free lime in the drilling fluids may range from about 1 to about 10 lbs/bbl, and more preferably about 1 to about 4 lbs/bbl, although lower ranges such as less than about 2 lbs/bbl are preferred for certain esters that tend to hydrolyze in the presence of alkaline compounds as will be known to those skilled in the art. Other suitable agents as an alternative to lime may also be used to adjust and/or stabilize invert emulsions of the drilling fluids with respect to acids. An example of such alternative agents is a protonated amine, as described in U.S. Pat. No. 5,977,031.

Further optional additives that may be present in the drilling fluids used in this invention include electrolytes, such as calcium chloride, organophilic bentonite and organophilic lignite. Glycols and/or glycerol may also be added. Still further, dispersion aids, corrosion inhibitors and/or defoamers may be used. These and other suitable auxiliaries and additives are used in amounts known to those skilled in the art depending on the conditions of the particular wellbore and subterranean formation.

In an alternative embodiment of the present invention, in addition to the non-ionic surfactant thinners of the present invention described above, additional thinners may be added advantageously in combination with said non-ionic surfactant thinners. Such particularly advantageous co-thinners are alkoxylated compounds of the general formula (V): R″″—(C₂H₄O)_(n)(C₃H₆O)_(m)(C₄H₈O)_(k)—H  (V) where R″″ is a saturated or unsaturated, linear or branched, alkyl radical having about 8 to about 24 carbon atoms, n is a number ranging from about 1 to about 10, m is a number ranging from about 0 to about 10, and k is a number ranging from about 0 to about 10. Preferably, R″″ has about 8 to about 18 carbon atoms; more preferably, R″″ has about 12 to about 18 carbon atoms; and most preferably, R″″ has about 12 to about 14 carbon atoms. Also, most preferably, R″″ is saturated and linear.

The compositions or compounds of formula (V) may be prepared by customary techniques of alkoxylation, such as alkoxylating the corresponding fatty alcohols with ethylene oxide and/or propylene oxide or butylene oxide under pressure and in the presence of acidic or alkaline catalysts as is known in the art. Such alkoxylation may take place blockwise, i.e., the fatty alcohol may be reacted first with ethylene oxide, propylene oxide or butylene oxide and subsequently, if desired, with one or more of the other alkylene oxides. Alternatively, such alkoxylation may be conducted randomly, in which any desired mixture of ethylene oxide, propylene oxide and/or butylene oxide is reacted with the fatty alcohol.

In formula (V), the subscripts n and m respectively represent the number of ethylene oxide (EO) and propylene oxide (PO) molecules or groups in one molecule of the alkoxylated fatty alcohol. The subscript k indicates the number of butylene oxide (BO) molecules or groups. The subscripts n, m, and k need not be integers, since they indicate in each case statistical averages of the alkoxylation. Included without limitation are those compounds of the formula (V) whose ethoxy, propoxy, and/or butoxy group distribution is very narrow, such as for example, “narrow range ethoxylates” also called “NREs” by those skilled in the art.

To accomplish the purposes of this invention, the compound of formula (V) must contain at least one ethoxy group and may have up to or about 10 ethoxy groups. Preferably, the compound of formula (V) will also contain at least one propoxy group (C₃H₆O—) or butoxy group (C₄H₈O—). Mixed alkoxides containing all three alkoxide groups—ethylene oxide, propylene oxide, and butylene oxide—are possible for the invention but are not preferred.

Preferably, for use according to this invention, the compound of formula (V) will have a value for m ranging from about 1 to about 10 with k zero or a value for k ranging from about 1 to about 10 with m zero. Most preferably, m will be about 1 to about 10 and k will be zero.

Other preferred compounds for use in the invention having the formula (V) above will have n ranging from about 1 to about 6, m ranging from about 1 to about 6, and k zero. Still other preferred compounds for use in the invention having the formula (V) above will have n ranging from about 2 to about 5, and m being about 3 or about 4 with k zero. It is particularly advantageous to establish the distribution of ethylene oxide and propylene oxide groups in the compounds of formula (V) in an ethylene oxide to propylene oxide ratio of about 1:1 to about 2:1, or even more preferably, about 2:1.5.

Additional preferred compounds for use in the invention having formula (V) above will have alkyl radicals containing about 12 to about 18 carbon atoms, or more preferably about 12 to about 14 carbon atoms, with subscripts n and m each having values of about 4 or about 5.

Both the non-ionic surfactant thinners of the present invention and the thinners of formula (V) may be added to the drilling fluid (or well service fluid) during initial preparation of the fluid or later as the fluid is being used for drilling or well service purposes in the formation. Alternatively, the non-ionic surfactant thinners may be added first at either of these times and the thinners of formula (V) may be added later. In still another embodiment, the thinners of formula (V) may be added first during either of these times and the non-ionic surfactant thinners of the invention may be added later.

The quantity of thinners added is an effective amount to maintain or effect the desired viscosity of the drilling fluid, preferably or particularly over a broad temperature range. For purposes of this invention, an “effective amount” of non-ionic surfactant thinner is preferably from about 0.5 to about 15 pounds per barrel of drilling fluid or mud. A more preferred amount of surfactant thinner ranges from about 1 to about 5 pounds per barrel of drilling fluid and a most preferred amount is about 1.5 to about 3 pounds thinner per barrel of drilling fluid. When the non-ionic surfactant thinners are used with formula (V) thinners, it is preferred to use the non-ionic surfactant thinners of the present invention and the thinners of formula (V) in proportions of from about 1:1 to about 10:1.

The thinners of the present invention are biodegradable and are of little or no toxicity. They are expected to be capable of meeting increasingly stringent environmental regulations affecting the oil and gas industry worldwide.

Although the invention has primarily been described in the context of a method of using non-ionic surfactants alone and in combination with the compounds of formula (V) as thinners for drilling fluids over a broad temperature range, these non-ionic surfactants alone and in combination with compounds of formula (V) may also be effective as thinners for well service fluids such as spotting fluids or workover fluids over a broad temperature range.

Further description and use of the invention is shown by the following examples:

EXAMPLES

To show the effect of the invention, the following experiments were conducted. In each case an invert emulsion drilling mud system of the following general composition was prepared:

Ester Bbl 0.496 Water bbl 0.233 Emulsifier lb 6.0 Organophilic bentonite lb 1.0 Organophilic lignite lb 5.0 Alkali reserve (lime) lb 1.5 CaCl₂ × 2 H₂O lb 27.2 Barite lb 314.0 Rev. dust lb 45.5 Dispersing auxiliary lb 0.5 Thinner lb/bbl 3.0

The oil phase (A) used was a 2-ethylhexyl octanoate as disclosed in EP 0 386 636. The emulsifier used was the product EZ MUL NTE (Baroid Drilling Fluids Inc., Houston, Tex.). The oil/water ratio was 70/30 in each case. Measurements were carried out on a system without thinner (C1), and with four non-ionic surfactant thinners E1 to E4 of the invention.

E1 RS1100 ™, of Cognis, Germany, see below E2 like E1, but reacted with 39 parts of ethylene oxide E3 like E1, but reacted with no parts of ethylene oxide E4 like E3, but reacted with 25 parts of ethylene oxide E5 like E1, but reacted with 50 parts of ethylene oxide

E1 was prepared according to the procedure described in U.S. Pat. No. 5,237,080 to Dante et al., assigned to Henkel Kommandiengesellschaft auf Aktien, by reacting fatty acids (60% C₈, 35% C₁₀, AN=361.9) with soya oil epoxide and distilling the product to obtain a clear yellow polyol (viscosity=5550 mPas; 20° C.; OHN=105, SN=236, AN=3.1). This reaction product was then admixed with potassium hydroxide in methanol and heated, after which all traces of methanol were removed. The product was then reacted with 61 parts of ethylene oxide at a pressure not exceeding 5 bar to yield, after neutralization, a clear yellow liquid (OHN=54.7). This product may be obtained from Cognis, Germany, under the tradename RS 1100.

The invert drilling fluids or muds were prepared in a conventional manner and subsequently, at 40° F. and 120° F., the rheological characteristics of plastic viscosity (PV) and yield point (YP) and the gel strength after 10 seconds and 10 minutes using a Fann SR12 rheometer (from Fann) were determined.

The results of the measurements are given in Table 1:

TABLE 1 C1 C1 E1 E1 E2 E2 E3 E3 E4 E4 E5 E5 Temperature 40 120 40 120 40 120 40 120 40 120 40 20 ° F. PV 118 40 106 45 111 12 110 37 107 34 100 31 (cP) YP 38 14 32 10 26 35 53 20 21 7 15 4 lb/100 ft² Gels 16/22 6/11 7/8 3/5 13/22 5/8 20/30 8/4 9/14 4/5 6/10 3/5 10″/10′

The data, especially for the yield point (YP), clearly indicate the advantageous thinning effect of the non-ionic surfactant thinners used according to the invention.

Further experiments may be seen in Tables 2 to 17. In these cases, the yield point (YP) of the systems tested was investigated at different temperatures and depicted as a graph. The measurements were carried out using a Fann 35 viscometer (from Fann). The tables also indicate the dial readings at different speeds of rotation per minute (rpm).

In Tables 2 to 17:

-   -   PETROFREE LV® is 2-ethylhexyl octanoate (Cognis, Germany)     -   PETROFREE® is C8-14 fatty acid 2-ethylhexyl ester (Cognis)     -   GELTONE II® is organophilic bentonite (Baroid, Houston, Tex.)     -   Thinner E1 is RS 1100™ (Cognis)     -   E1, E2, E3, E4, and E5 are the same as indicated above and as in         Table 1.

TABLE 2 Mud system PETROFREE Mud weight, lb/gal 9.5 Oil/water ratio 70/30 E1, lb/bbl 0 1 Temperature, ° F. 40 120 40 120 Plastic viscosity, cP 75 26 78 23 Yield point, lb/100 ft² 62 31 29 15 10 sec gel, lb/100 ft² 26 16 23 8 10 min gel, lb/100 ft² 27 17 23 10 Fann 35 dial readings 600 rpm 212 83 185 61 300 rpm 137 57 107 38 200 rpm 110 47 79 30 100 rpm 77 36 49 20 6 rpm 30 18 14 9 3 rpm 26 16 11 7

TABLE 3 Mud system PETROFREE Mud weight, lb/gal 14.0 Oil/water ratio 75/25 Contaminant Excess GELTONE II E1 0 3 Temperature, ° F. 40 120 40 120 Plastic viscosity, cP 180 51 140 36 Yield point, lb/100 ft² 230 152 19 8 10 sec gel, lb/100 ft² 108 64 7 3 10 min gel, lb/100 ft² 110 66 13 4 Fann 35 dial readings 600 rpm 590 254 299 80 300 rpm 410 203 159 44 200 rpm 336 179 110 30 100 rpm 248 146 62 18 6 rpm 112 79 8 3 3 rpm 100 70 6 2

TABLE 4 Mud system PETROFREE Mud weight, lb/gal 11.0 Oil/water ratio 70/30 Contaminant Drill solids E1, lb/bbl 0 3 Temperature, ° F. 40 120 40 120 Plastic viscosity, cP 11 34 108 35 Yield point, lb/100 ft² 90 47 27 7 10 sec gel, lb/100 ft² 38 21 4 3 10 min gel, lb/100 ft² 44 24 7 5 Fann 35 dial readings 600 rpm 300 115 243 77 300 rpm 200 81 135 42 200 rpm 157 67 95 30 100 rpm 110 50 52 17 6 rpm 42 23 7 3 3 rpm 38 21 5 2

TABLE 5 Mud system PETROFREE Mud weight, lb/gal 11.0 Oil/water ratio 70/30 Contaminant Excess GELTONE II E1, lb/bbl 0 3 Temperature, ° F. 40 120 40 120 Plastic viscosity, cP 132 31 62 18 Yield point, lb/100 ft² 54 53 11 3 10 sec gel, lb/100 ft² 33 23 3 2 10 min gel, lb/100 ft² 38 27 5 3 Fann 35 dial readings 600 rpm 318 115 135 39 300 rpm 186 84 73 21 200 rpm 139 71 52 14 100 rpm 91 54 29 9 6 rpm 35 25 4 2 3 rpm 32 21 3 1

TABLE 6 Mud system PETROFREE LV Mud weight, lb/gal 14.0 Oil/water ratio 70/30 Contaminant Drill solids E1, lb/bbl 0 3 Temperature, ° F. 40 120 40 120 Plastic viscosity, cP 118 40 106 45 Yield point, lb/100 ft² 38 14 32 10 10 sec gel, lb/100 ft² 16 6 7 3 10 min gel, lb/100 ft² 22 11 8 5 Fann 35 dial readings 600 rpm 274 94 244 100 300 rpm 156 54 138 55 200 rpm 114 40 98 39 100 rpm 70 25 55 22 6 rpm 17 6 8 4 3 rpm 14 5 5 3

TABLE 7 Mud system PETROFREE LV Mud weight, lb/gal 14.0 Oil/water ratio 70/30 Contaminant Drill solids E1, lb/bbl 0 3 Temperature, ° F. 40 120 40 120 Plastic viscosity, cP 110 37 100 31 Yield point, lb/100 ft² 53 20 15 4 10 sec gel, lb/100 ft² 20 8 6 3 10 min gel, lb/100 ft² 30 14 10 5 Fann 35 dial readings 600 rpm 273 94 215 66 300 rpm 163 57 115 35 200 rpm 124 42 80 25 100 rpm 80 27 44 15 6 rpm 23 8 7 3 3 rpm 19 7 5 2

TABLE 8 Mud system PETROFREE Mud weight, lb/gal 9.5 Oil/water ratio 70/30 E5, lb/bbl 0 3 Temperature, ° F. 120 120 Plastic viscosity, cP 22 21 Yield point, lb/100 ft² 39 16 10 sec gel, lb/100 ft² 16 8 10 min gel, lb/100 ft² 17 10 Fann 35 dial readings 600 rpm 83 58 300 rpm 61 37 200 rpm 52 28 100 rpm 40 20 6 rpm 19 8 3 rpm 17 7

TABLE 9 Mud system PETROFREE LV Mud weight, lb/gal 9.5 Oil/water ratio 70/30 Contaminant Drill solids E1, lb/bbl 0 3 Temperature, ° F. 40 120 40 120 Plastic viscosity, cP 65 20 52 20 Yield point, lb/100 ft² 31 33 27 17 10 sec gel, lb/100 ft² 29 17 17 10 10 min gel, lb/100 ft² 32 22 26 15 Fann 35 dial readings 600 rpm 151 73 131 57 300 rpm 96 53 79 37 200 rpm 78 44 66 30 100 rpm 57 34 29 20 6 rpm 29 18 14 9 3 rpm 27 16 12 8

TABLE 10 Mud system PETROFREE Mud weight, lb/gal 14.0 Oil/water ratio 75/25 Contaminant Excess GELTONE II E5, lb/bbl 0 3 Temperature, ° F. 40 120 40 120 Plastic viscosity, cP 180 51 175 47 Yield point, lb/100 ft² 230 152 34 10 10 sec gel, lb/100 ft² 108 64 10 3 10 min gel, lb/100 ft² 110 66 14 5 Fann 35 dial readings 600 rpm 590 254 384 104 300 rpm 410 203 209 57 200 rpm 336 179 148 40 100 rpm 248 146 82 23  6 rpm 112 79 12 3  3 rpm 100 70 9 2

TABLE 11 Mud system PETROFREE Mud weight, lb/gal 11.0 Oil/water ratio 70/30 Contaminant Drill solids E5, lb/bbl 0 3 Temperature, ° F. 40 120 40 120 Plastic viscosity, cP 110 34 106 30 Yield point, lb/100 ft² 90 47 20 9 10 sec gel, lb/100 ft² 38 21 8 5 10 min gel, lb/100 ft² 44 24 11 9 Fann 35 dial readings 600 rpm 310 115 232 69 300 rpm 200 81 126 39 200 rpm 157 67 88 28 100 rpm 110 50 49 17  6 rpm 42 23 9 5  3 rpm 38 21 7 4

TABLE 12 Mud system PETROFREE Mud weight, lb/gal 11.0 Oil/water ratio 70/30 Contaminant Excess GELTONE II E5, lb/bbl 0 3 Temperature, ° F. 40 120 40 120 Plastic viscosity, cP 132 31 90 23 Yield point, lb/100 ft² 54 53 20 9 10 sec gel, lb/100 ft² 33 23 6 2 10 min gel, lb/100 ft² 38 27 9 3 Fann 35 dial readings 600 rpm 318 115 200 55 300 rpm 186 84 110 32 200 rpm 139 71 77 22 100 rpm 91 54 42 13  6 rpm 35 25 6 2  3 rpm 32 21 4 1

TABLE 13 Mud system PETROFREE LV Mud weight, lb/gal 14.0 Oil/water ratio 70/30 Contaminant Drill Solids E1, lb/bbl 0 1 E2, lb/bbl 0 2 Temperature, ° F. 40 120 40 120 Plastic viscosity, cP 118 40 105 33 Yield point, lb/100 ft² 38 14 15 6 10 sec gel, lb/100 ft² 16 6 7 3 10 min gel, lb/100 ft² 22 11 12 6 Fann 35 dial readings 600 rpm 274 94 225 72 300 rpm 156 54 120 39 200 rpm 114 40 83 28 100 rpm 70 25 45 16  6 rpm 17 6 7 3  3 rpm 14 5 5 2

TABLE 14 Mud system PETROFREE LV Mud weight, lb/gal 14.0 Oil/water ratio 70/30 Contaminant Drill solids (5%) E2, lb/bbl 0 3 5 Temperature, ° F. 40 120 40 120 40 120 Plastic viscosity, cP 118 40 111 35 114 37 Yield point, lb/100 ft² 38 14 26 7 17 2 10 sec gel, lb/100 ft² 16 6 13 5 6 2 10 min gel, lb/100 ft² 22 11 22 8 9 4 Fann 35 dial readings 600 rpm 274 94 248 77 245 76 300 rpm 156 54 137 42 131 39 200 rpm 114 40 98 30 91 27 100 rpm 70 25 57 18 50 16  6 rpm 17 6 12 4 6 2  3 rpm 14 5 10 3 4 1

TABLE 15 Mud system PETROFREE LV Mud weight, lb/gal 14.0 Oil/water ratio 70/30 Contaminant Drill solids (10%) E2, lb/bbl 0 5 7 Temperature, ° F. 40 120 40 120 40 120 Plastic viscosity, cP 176 59 152 48 150 47 Yield point, lb/100 ft² 48 19 14 6 20 4 10 sec gel, lb/100 ft² 16 9 8 6 8 3 10 min gel, lb/100 ft² 28 17 13 8 10 4 Fann 35 dial readings 600 rpm 400 137 318 104 320 98 300 rpm 224 78 166 56 170 51 200 rpm 170 57 115 40 118 36 100 rpm 100 34 62 23 64 20  6 rpm 18 8 8 4 8 3  3 rpm 14 7 7 3 6 2

TABLE 16 Mud system PETROFREE LV Mud weight, lb/gal 14.0 Oil/water ratio 70/30 Contaminant Drill Solids E3, lb/bbl 0 3 Temperature, ° F. 40 120 40 120 Plastic viscosity, cP 118 40 115 36 Yield point, lb/100 ft² 38 14 35 14 10 sec gel, lb/100 ft² 16 6 25 6 10 min gel, lb/100 ft² 22 11 30 9 Fann 35 dial readings 600 rpm 274 94 265 86 300 rpm 156 54 150 50 200 rpm 114 40 110 37 100 rpm 70 25 65 23  6 rpm 17 6 17 5  3 rpm 14 5 13 4

TABLE 17 Mud system PETROFREE LV Mud weight, lb/gal 14.0 Oil/water ratio 70/30 Contaminant Drill Solids E3, lb/bbl 0 3 Temperature, ° F. 40 120 40 120 Plastic viscosity, cP 118 40 115 36 Yield point, lb/100 ft² 38 14 35 14 10 sec gel, lb/100 ft² 16 6 25 6 10 min gel, lb/100 ft² 22 11 30 9 Fann 35 dial readings 600 rpm 274 94 265 86 300 rpm 156 54 150 50 200 rpm 114 40 110 37 100 rpm 70 25 65 23  6 rpm 17 6 17 5  3 rpm 14 5 13 4

The foregoing description of the invention is intended to be a description of preferred embodiments. Various changes in the details of the described compositions and method can be made without departing from the intended scope of this invention as defined by the appended claims. 

1. A method of drilling a borehole in a subterranean formation at a temperature in the range of about 40° F. to about 250° F., comprising: providing or using in the drilling an invert emulsion drilling fluid comprising a continuous oil phase and having a plastic viscosity in the range of about 10 cP to about 60 cP and a yield point in the range of about 5 lb/100 ft² to about 40 lb/100 ft² when measured at about 122° F. (50° C.) and a yield point in the range of about 5 lb/100 ft² to about 75 lb/100 ft² when measured at about 40° F. (4° C.); and adding to the drilling fluid before or during the drilling at least one non-ionic surfactant thinner, wherein the surfactant is the reaction product of at least one oxide, selected from the group consisting of ethylene oxide, propylene oxide and butylene oxide, with C₁₀₋₂₂ carboxylic acids or C₁₀₋₂₂ carboxylic acid derivatives containing at least one double bond in position 9, 10, 13, or 14 having structural units of the general formula (I):

where R₁ is a hydrogen atom, or an OH group, or a group OR₂, and where R₂ is an alkyl group of about 1 to about 18 carbon atoms, or an alkenyl group of about 2 to about 18 carbon atoms, or a group of the formula (II):

and where R₃ is a hydrogen atom, or an alkyl group of about 1 to about 21 carbon atoms, or an alkylene group of about 2 to about 21 carbon atoms; such that the drilling fluid maintains during the drilling a plastic viscosity in the range of about 10 cP to about 60 cP and a yield point in the range of about 5 lb/100 ft² to about 40 lb/100 ft² when measured at about 122° F. (50° C.) and a yield point in the range of about 5 lb/100 ft² to about 75 lb/100 ft² when measured at about 40° F. (4° C.).
 2. The method of claim 1 wherein the surfactant thinner comprises alkylation products obtained by reacting C₁₂₋₂₂ carboxylic mono-, di- or triglycerides containing structural units of the formula (I) in at least one position 9, 10, 13, or 14 with at least one oxide of the group comprising ethylene oxide, propylene oxide, and butylene oxide.
 3. The method of claim 1 wherein the surfactant thinner thins the drilling fluid so that the continuous oil phase of the fluid is flowable and pumpable at temperatures at least as low as about 40° F. (40° C.).
 4. The method of claim 1 wherein the surfactant thinner thins the drilling fluid so that the continuous oil phase of the fluid is flowable and pumpable at temperatures at least as low as about 32° F. (0° C.).
 5. The method of claim 1 wherein the continuous oil phase comprises compounds or compositions selected from the group consisting of: (a) carboxylic esters of the formula: R′—COO—R″ where R′ is a saturated or unsaturated, linear or branched, alkyl radical having about 1 to about 23 carbon atoms and R″ is an alkyl radical, branched or unbranched, saturated or unsaturated, having about 1 to about 23 carbon atoms; (b) linear or branched olefins having about 8 to about 30 carbon atoms; (c) water-insoluble symmetric or asymmetric ethers of monohydric alcohols of natural or synthetic origin, said alcohols containing about 1 to about 24 carbon atoms; (d) water-insoluble alcohols of the formula: R′″—OH where R′″ is a saturated, unsaturated, linear or branched alkyl radical having about 8 to about 24 carbon atoms; and (e) carbonic diesters.
 6. The method of claim 1 wherein the drilling fluid further comprises drilling fluid water dispersed in the oil phase and solids insoluble in the oil phase.
 7. The method of claim 1 further comprising adding to the drilling fluid before or during the drilling a co-thinner compound having the formula: R—(C₂H₄O)_(n)(C₃H₆O)_(m)(C₄H₈O)_(k)—H where R is a saturated or unsaturated, linear or branched alkyl radical having about 8 to about 24 carbon atoms, n is a number ranging from about 1 to about 10, m is a number ranging from about 0 to about 10, and k is a number ranging from about 0 to about
 10. 8. The method of claim 7 wherein in the formula for the co-thinner compound, k is zero and m is a number ranging from about 1 to about 10, or m is zero and k is a number ranging from about 1 to about
 10. 9. The method of claim 7 wherein in the formula for the co-thinner compound, n is a number ranging from about 1 to about 6, m is a number ranging from about 1 to about 6, and k is zero.
 10. The method of claim 7 wherein the surfactant thinner and the co-thinner compound are added to the drilling fluid in proportions ranging from about 1:1 to about 10:1.
 11. The method of claim 10 wherein the surfactant thinner is added to the drilling fluid during preparation of the fluid and the co-thinner is added to the drilling fluid during drilling.
 12. The method of claim 10 wherein the surfactant thinner and the co-thinner are both added to the drilling fluid during preparation of the fluid.
 13. The method of claim 10 wherein the surfactant thinner and the co-thinner are both added to the drilling fluid during drilling.
 14. The method of claim 1 wherein the surfactant thinner is added to the drilling fluid in quantities ranging from about 0.5 pounds to about 15.0 pounds per barrel of the drilling fluid.
 15. The method of claim 1 wherein the surfactant thinner is added to the drilling fluid during drilling in sufficient amounts to reduce the viscosity of the fluid during drilling such that the plastic viscosity of the fluid is maintained during drilling in the range of about 10 cP to about 60 cP, the yield point of the fluid is maintained during drilling in the range of about 5 lb/100 ft² to about 40 lb/100 ft² when measured at about 122° F. (50° C.), and the yield point of the fluid is maintained during drilling in the range of about 5 lb/100 ft² to about 75 lb/100 ft² when measured at about 40° F. (4° C.).
 16. A drilling fluid comprising: a continuous oil phase; water dispersed in the oil phase; solids insoluble in the oil phase; and at least one non-ionic surfactant thinner, wherein the non-ionic surfactant thinner is the reaction product of ethylene oxide, propylene oxide and/or butylene oxide with C₁₀₋₂₂ carboxylic acids or C₁₀₋₂₂ carboxylic acid derivatives containing at least one double bond in position 9, 10, 13, or 14 having structural units of the general formula (I):

where R₁ is a hydrogen atom, or an OH group, or a group OR₂, and where R₂ is an alkyl group of about 1 to about 18 carbon atoms, or an alkenyl group of about 2 to about 18 carbon atoms, or a group of the formula (II):

where R₃ is a hydrogen atom, or an alkyl group of about 1 to about 21 carbon atoms, or an alkylene group of about 2 to about 21 carbon atoms; and wherein the surfactant thinner is added to the fluid in quantities ranging from about 0.5 pounds to about 15.0 pounds of surfactant per barrel of fluid, such that the fluid has a plastic viscosity in the range of about 10 cP to about 60 cP and a yield point in the range of about 5 lb/100 ft² to about 40 lb/100 ft² when measured at about 122° F. (50° C.), and a yield point in the range of about 5 lb/100 ft² to about 75 lb/100 ft² when measured at about 40° F. (4° C.).
 17. The drilling fluid of claim 16 wherein the continuous oil phase comprises compounds or compositions selected from the group consisting of: (a) carboxylic esters of the formula: R′—COO—R″ where R′ is a saturated or unsaturated, linear or branched, alkyl radical having about 1 to about 23 carbon atoms and R″ is an alkyl radical, branched or unbranched, saturated or unsaturated, having about 1 to about 23 carbon atoms; (b) linear or branched olefins having about 8 to about 30 carbon atoms; (c) water-insoluble symmetric or asymmetric ethers of monohydric alcohols of natural or synthetic origin, said alcohols containing about 1 to about 24 carbon atoms; (d) water-insoluble alcohols of the formula: R′″—OH where R′″ is a saturated, unsaturated, linear or branched alkyl radical having about 8 to about 24 carbon atoms; and (e) carbonic diesters.
 18. The drilling fluid of claim 17 wherein one or more compounds or compositions from said group comprise about 50% to about 100% of the oil phase by volume.
 19. The drilling fluid of claim 16 wherein the drilling fluid comprises about 15% to about 35% by volume water and about 75% to about 65% by volume oil.
 20. The drilling fluid of claim 16 wherein the quantity of surfactant thinner in the drilling fluid ranges from about 1.5 lbs/barrel to about 3 lbs/barrel.
 21. The drilling fluid of claim 16 further comprising a co-thinner compound having the formula: R—(C₂H₄O)_(n)(C₃H₆O)_(m)(C₄H₈O)_(k)—H where R is a saturated or unsaturated, linear or branched alkyl radical having about 8 to about 24 carbon atoms, n is a number ranging from about 1 to about 10, m is a number ranging from about 0 to about 10, and k is a number ranging from about 0 to about 10, and wherein the surfactant thinner and the co-thinner compound are present in the drilling fluid in a ratio ranging from about 1:1 to about 10:1. 